1. Field of the Invention
This invention relates to an electrical machine with a rotor and a stator. The rotor is effectively connected with a rotor shaft that is rotationally mounted in a housing by means of at least one bearing, such as a roller bearing.
2. Technical Considerations
Electrical machines are known in the form of direct-current machines or three-phase machines (asynchronous or synchronous machines). For cooling direct-current motors, an air cooling system is frequently provided in which a fan impeller driven by the rotor shaft generates an air current through the housing of the machine.
Three-phase machines generally have a closed housing and a small air gap between the stator and the rotor. This means that it is practically impossible to cool these machines with air. On three-phase machines of the known art, the housing is closed for maintenance reasons and is provided with fins on the outside to discharge heat.
As the power of electrical machines increases, such cooling systems are no longer able to discharge a sufficient amount of heat.
This problem is present to a particular degree with drive axles in which one or more electrical machines are installed. As a result of which, the electrical machines reach high steady-state temperatures. In work machines realized in the form of industrial trucks that are operated in multiple-shift operations, for example, thermal overloads can occur in the problem zones in the vicinity of the bearings and the sealing devices of the electrical machines. This can lead to the failure of the sealing devices or of the bearings.
In comparison to air cooling systems, liquid cooling systems are significantly more efficient and make it possible to discharge a large amount of heat. So, to achieve the same power output, the size of the electrical machine can be reduced, or for an electrical machine of the same size, the power output can be increased.
On three-phase machines, liquid cooling systems are known in which a system of tubes to cool the stator is located on the outer jacket. However, an external cooling system to cool the entire three-phase machine is difficult and expensive to construct.
Electrical machines with an internal liquid cooling system are also known in which the rotor runs under oil, although that causes increased splash losses.
A high cooling capacity is also required in such internal and external liquid cooling systems because the entire electrical machine must be cooled, which increases the complexity and expense of construction.
Therefore, it is an object of the invention to provide an electrical machine of the general type described above but which makes possible an efficient cooling and is easy and economical to construct.